The subject matter disclosed herein generally relates to monitoring a catalyst in an engine system and more particularly to methods and systems for continuously diagnosing a three way catalyst and taking corrective control action in the case of catalyst deactivation.
Environmental regulations require the use of catalysts to treat engine exhaust in order to reduce air pollution. A catalytic converter uses two types of catalysts, a reduction catalyst and an oxidation catalyst. The catalytic converter consists of a ceramic structure coated with a metal catalyst incorporated within a housing. The catalytic converter provides a structure that exposes the maximum surface area of catalyst to the exhaust stream.
A three-way catalytic converter has the capacity to store oxygen (O2). When the air/fuel ratio of the exhaust is lean (oxidizing atmosphere), it stores O2 and thereby suppresses the production of mono-nitrogen oxides (NOx). When the air/fuel ratio of the exhaust is rich, it releases the stored O2 thereby accelerating the oxidation of hydrocarbons (HC) and carbon monoxide (CO).
In many applications, it is desirable to monitor the performance of the catalytic converter. Failure to detect catalyst deactivation on gas engines might result in severe financial penalties for the end-user. Monitoring may involve sensing the exhaust gases to determine whether the catalyst is performing adequately. Among the sensors used are O2 sensors and NOx sensors. In the case of O2, the sensors may be located upstream and downstream of the catalyst. Signals from the sensors are compared and correlated to the emissions to determine whether the catalyst is performing adequately.
Another approach to monitoring the performance of a catalytic converter is to sense the temperature of the catalytic converter. Usually two sensors will be fitted. One sensor is disposed upstream from the catalyst and the other sensor is disposed downstream from the catalyst. The sensors monitor the temperature rise over the catalytic converter core. When the temperature difference between the sensors is greatest, the catalytic converter is thought to be working optimally.
For a system having a three way catalyst combined with an ammonia slip catalyst and mid bed air injection, the engine is typically run with a rich air/fuel ratio. Running the engine rich achieves the benefit of reducing NOx in the three way catalyst and oxidizing CO and NH3 in the ammonia slip catalyst. Three way catalysts lose performance when chemically deactivated. For example, oil exposure for a 4000 hour duration may chemically deactivate a three way catalyst. Such deactivation could result in CO and methane (CH4) emissions increasing while NH3 emissions decrease which could result in the engines being out of compliance with environmental regulations. Prior methodologies for monitoring catalytic converter performance are typically based on catalyst temperature and O2 storage based diagnosis. As the engine is typically run rich in a mid-bed air injection scenario, O2 storage based diagnosis will not be valid and hence, there is a need for an alternate way of monitoring the catalyst health.